It has always intrigued me as to whether smoke deflectors actually worked. From a layman perspective I'd say they worked pretty well for most VR locos. They were virtually redundant on NSWGR locos and they worked for some of SAR locos but not all. I'm not sure of WAGR and QR locos of the era.
Does anyone have the answer as to why there was disparity?
Smoke deflectors were intended to keep smoke clear of the cab to reduce the chances of the smoke obscuring the driver's view.
These were introduced in Victoria after Brownbill's work on modified draughting was adopted for most locomotives, because the new arrangements reduced the energy in the blast pipe while reducing back pressure and increasing power.
So drifting exhaust became a more serious problem on the VR on locomotives with modified draughting and smoke deflectors were introduced.
Even the VR were not completely consistent. While all of the K class were fitted with smoke deflectors, the D3 was not fitted, although both types had the modified grate and blast pipe and the boiler was almost identical.
When VR locomotives were fitted with cross compound air pumps (mainly the K and the N) the smoke deflector on the fireman's side was cut back to about a third of its length, becoming more or less a dummy.
On the SAR, only the 600 and 620 class were fitted. In the case of the 620 the streamlined casing affected the airflow and required the deflectors. (Of course the 750 class, ex VR N class, came with smoke deflectors.)
The NSWGR tested smoke deflectors in the 1930s on 35 and 36 class locomotives but didn't think they were justified.